Mixing-driven changes in distributions and abundances of planktonic microorganisms in a large, oligotrophic lake

Temperate lakes experience variation in mixing and stratification that affects the distributions, activities, abundances, and diversity of plankton communities. We examined temporal and vertical changes in the composition of planktonic microorganisms (including Bacteria and Archaea) in oligotrophic Flathead Lake, Montana. Using a combination of approaches that included 16S rRNA gene sequencing and flow cytometric determination of cell abundances, we found that the microbial community was responsive to variations in stratification and mixing at time scales ranging from episodic (scale of days) to seasonal. However, the impact of such physical dynamics varied among taxa, likely reflecting taxa-specific responses to environmental changes that coincide with stratification and mixing (e.g., light availability and nutrient supply). During the early spring, periods of relatively short-term (< 7 d) intermittency in stratification and mixing influenced the vertical distributions of specific microbial taxa, notably including the cyanobacteria. These events highlight time scales of biological responses to high-frequency variations associated with lake stratification and mixing, particularly during the transition to the growing season in the early spring.